The present invention relates to the preparation of high viscosity polyalphaolefins prepared using an ionic liquid catalyst.
Alphaolefins may be oligomerized to prepare synthetic lubricating oil base stocks which have desirable lubricating properties such as a low pour point and a high viscosity index (VI). However, very high viscosity polyalphaolefins, such as disclosed in U.S. Pat. No. 4,827,064, are expensive to manufacture using conventional oligomerization processes. U.S. Pat. No. 5,304,615 teaches the oligomerization of butene using an ionic liquid catalyst. European Patent Application 97300875.8 describes a process for oligomerizing alphaolefins, such as decene, using an ionic liquid catalyst to produce polyalphaolefins having a viscosity up to about 20 centistokes (cSt) at 100xc2x0 C. Unfortunately, the process taught in this application has not been shown to be suitable for making very high viscosity material, i.e., polyalphaolefins having a viscosity above 22 cSt at 100xc2x0 C.
Additionally, the prior art teaches the use of imidazolium, pyridinium, or phosphorium as one component in the ionic liquid in addition to aluminum halide or gallium halide. Ternary compositions with ammonium halides are described in WO 95/121872 as being useful for olefinic oligomerization.
Applicants have found that it is possible to readily make polyalphaolefins having very high viscosity using an ionic liquid catalyst by carrying out the oligomerization reaction in the absence of organic solvents which have hitherto been used as a diluent for the feed. Accordingly, Applicants have been able to make polyalphaolefins from feeds comprised primarily of olefins, such as decene and dodecene, having viscosities in excess of 22 cSt and even in excess of 30 cSt. Polyalphaolefins made using the process of the present invention also have been shown to display excellent viscosity index (VI) values, low pour points, and low Noack volatility values. As used in this disclosure, the words xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase xe2x80x9cconsists essentially ofxe2x80x9d or xe2x80x9cconsisting essentially ofxe2x80x9d is intended to mean the exclusion of other elements of any essential significance to the combination. The phrase xe2x80x9cconsisting ofxe2x80x9d is intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
The present invention is directed to a process for producing a very high viscosity polyalphaolefin product comprising contacting a feed consisting essentially of at least one alphaolefin having from 4 to about 14 carbon atom with an effective oligomerizing amount of an acidic ionic liquid oligomerization catalyst, maintaining said feed and oligomerization catalyst Under preselected oligomerization conditions for a sufficient time to oligomerize the alphaolefin to the polyalphaolefin product, and recovering the high viscosity polyalphaolefin product. As noted above, it has been found that very high viscosity products may be obtained using the process of the present invention by carrying out the oligomerization reaction in the absence of organic diluent. Using the process of the invention, polyalphaolefins having viscosities in excess of 22 cSt and even in excess of 30cSt may be readily prepared. Especially preferred in preparing the polyalphaolefin product are feeds comprising decene or dodecene.
The acidic ionic liquid oligomerization catalyst usually will be comprised of at least two components, and in most instances it will be a binary catalyst, i.e., it will consist of only two components. The first component is a compound selected from the group consisting of aluminum halide, alkyl aluminum halide, gallium halide, and alkyl gallium halide. Preferred compounds for use as the first component of the oligomerization catalyst are an aluminum halide or an alkyl aluminum halide, such as, for example, aluminum trichloride. The second component is quaternary ammonium, quaternary phosporium, or tertiary sulfonium, such as, for example, a liquid salt selected from one or more of hydrocarbyl substituted ammonium halides, hydocarbyl substituted imidazolium halide, hydrocarbyl substituted pyridinium halide, alkylene substituted pyridinium dihalide, or hydrocarbyl substituted phosphonium halide. Particularly preferred as the second component are alkyl substituted ammonium halides, such as trimethylamine hydrochloride or alkyl substituted imidizolium halides, such as 1-ethyl-3-methyl-imidazolium chloride. The mole ratio of the two components will usually fall within the range of from about 1:1 to about 5:1 of said first component to said second component, and more preferably the mole ratio will be in the range of from about 1:1 to about 2:1.
The use of a binary catalyst composition consisting essentially of trimethylamine hydrochloride and aluminum trichloride is particularly advantageous for carrying out the process of the present invention due to the ease of preparation, the ready commercial availability of the components, and the relatively low cost.
The amount of catalyst present to promote the oligomerization of the alphaolefin should be not less than an effective oligomerizing amount, that is to say, the minimum amount of the catalyst necessary to olgomerize the alphaolefin to the desired product. This may vary to some degree depending on the composition of the catalyst, the ratio of the two components of the catalyst to one another, the feed, the oligomerzation conditions chosen, and the like. However, a determination of the effective catalytic amount should be well within the ability of one skilled in the art with no more than routine testing necessary to establish the amount needed to carry out the invention. The present invention is also directed to the unique polyalphaolefin product prepared using the present invention. This product is characterized by a viscosity of not less than 22 cSt at 1 00xc2x0 C., and more preferably will have a viscosity of at least 30 cSt at 100xc2x0 C. In addition, the polyalphaolefin product will display a low pour point, preferably less than xe2x88x92300xc2x0 C., and low volatility, preferably with a Noack number of 3 or less. Preferably, the product will have a dimer content of less than 2 weight percent.